System providing ventricular pacing and biventricular coordination

ABSTRACT

A cardiac rhythm management system includes techniques for computing an indicated pacing interval, AV delay, or other timing interval. In one embodiment, a variable indicated pacing interval is computed based at least in part on an underlying intrinsic heart rate. The indicated pacing interval is used to time the delivery of biventricular coordination therapy even when ventricular heart rates are irregular, such as in the presence of atrial fibrillation. In another embodiment, a variable filter indicated AV interval is computed based at least in part on an underlying intrinsic AV interval. The indicated AV interval is used to time the delivery of atrial tracking biventricular coordination therapy when atrial heart rhythms are not arrhythmic. Other indicated timing intervals may be similarly determined. The indicated pacing interval, AV delay, or other timing interval can also be used in combination with a sensor indicated rate indicator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending, commonlyassigned patent applications: “Method and Apparatus for TreatingIrregular Ventricular Contractions Such as During Atrial Arrhythmia,”U.S. Ser. No. 09/316,515, “Cardiac Rhythm Management System PromotingAtrial Pacing,” U.S. Ser. No. 09/316,682, and “Cardiac Rhythm ManagementSystem With Atrial Shock Timing Optimization,” U.S. Ser. No. 09/316,741,each of which are filed on even date herewith, each of which disclosureis herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present system relates generally to cardiac rhythm managementsystems and particularly, but not by way of limitation, to a systemproviding, among other things, ventricular pacing and biventricularcoordination therapy.

BACKGROUND

When functioning properly, the human heart maintains its own intrinsicrhythm, and is capable of pumping adequate blood throughout the body'scirculatory system. However, some people have irregular cardiac rhythms,referred to as cardiac arrhythmias. Such arrhythmias result indiminished blood circulation. One mode of treating cardiac arrhythmiasuses drug therapy. Drugs are often effective at restoring normal heartrhythms. However, drug therapy is not always effective for treatingarrhythmias of certain patients. For such patients, an alternative modeof treatment is needed. One such alternative mode of treatment includesthe use of a cardiac rhythm management system. Such systems are oftenimplanted in the patient and deliver therapy to the heart.

Cardiac rhythm management systems include, among other things,pacemakers, also referred to as pacers. Pacers deliver timed sequencesof low energy electrical stimuli, called pace pulses, to the heart, suchas via an intravascular leadwire or catheter (referred to as a “lead”)having one or more electrodes disposed in or about the heart. Heartcontractions are initiated in response to such pace pulses (this isreferred to as “capturing” the heart). By properly timing the deliveryof pace pulses, the heart can be induced to contract in proper rhythm,greatly improving its efficiency as a pump. Pacers are often used totreat patients with bradyarrhythmias, that is, hearts that beat tooslowly, or irregularly.

Cardiac rhythm management systems also include cardioverters ordefibrillators that are capable of delivering higher energy electricalstimuli to the heart. Defibrillators are often used to treat patientswith tachyarrhythmias, that is, hearts that beat too quickly. Suchtoo-fast heart rhythms also cause diminished blood circulation becausethe heart isn't allowed sufficient time to fill with blood beforecontracting to expel the blood. Such pumping by the heart isinefficient. A defibrillator is capable of delivering an high energyelectrical stimulus that is sometimes referred to as a defibrillationcountershock. The countershock interrupts the tachyarrhythmia, allowingthe heart to reestablish a normal rhythm for the efficient pumping ofblood. In addition to pacers, cardiac rhythm management systems alsoinclude, among other things, pacer/defibrillators that combine thefunctions of pacers and defibrillators, drug delivery devices, and anyother implantable or external systems or devices for diagnosing ortreating cardiac arrhythmias.

One problem faced by cardiac rhythm management systems is the treatmentof congestive heart failure (also referred to as “CHF”). Congestiveheart failure, which can result from long-term hypertension, is acondition in which the muscle in the walls of at least one of the rightand left sides of the heart deteriorates. By way of example, suppose themuscle in the walls of left side of the heart deteriorates. As a result,the left atrium and left ventricle become enlarged, and the heart muscledisplays less contractility. This decreases cardiac output of bloodthrough the circulatory system which, in turn, may result in anincreased heart rate and less resting time between heartbeats. The heartconsumes more energy and oxygen, and its condition typically worsensover a period of time.

In the above example, as the left side of the heart becomes enlarged,the intrinsic electrical heart signals that control heart rhythm arealso affected. Normally, such intrinsic signals originate in thesinoatrial (SA) node in the upper right atrium, traveling through anddepolarizing the atrial heart tissue such that resulting contractions ofthe right and left atria are triggered. The intrinsic atrial heartsignals are received by the atrioventricular (AV) node which, in turn,triggers a subsequent ventricular intrinsic heart signal that travelsthrough and depolarizes the ventricular heart tissue such that resultingcontractions of the right and left ventricles are triggeredsubstantially simultaneously.

In the above example, where the left side of the heart has becomeenlarged due to congestive heart failure, however, the ventricularintrinsic heart signals may travel through and depolarize the left sideof the heart more slowly than in the right side of the heart. As aresult, the left and right ventricles do not contract simultaneously,but rather, the left ventricle contracts after the right ventricle. Thisreduces the pumping efficiency of the heart. Moreover, in the case ofleft bundle branch block (LBBB), for example, different regions withinthe left ventricle may not contract together in a coordinated fashion.

Congestive heart failure can be treated by biventricular coordinationtherapy that provides pacing pulses to both right and left ventricles.See, e.g., Mower U.S. Pat. No. 4,928,688. Congestive heart failure mayalso result in an overly long atrioventricular (AV) delay between atrialand ventricular contractions, again reducing the pumping efficiency ofthe heart. There is a need to provide congestive heart failure patientswith improved pacing and coordination therapies for improving the AVdelay, coordinating ventricular contractions, or otherwise increasingheart pumping efficiency.

Another problem faced by cardiac rhythm management systems is thepresence of atrial tachyarrhythmias, such as atrial fibrillation,occurring in patients having congestive heart failure. Atrialfibrillation is a common cardiac arrhythmia that reduces the pumpingefficiency of the heart, though not to as great a degree as inventricular fibrillation. However, this reduced pumping efficiencyrequires the ventricle to work harder, which is particularly undesirablein congestive heart failure or other sick patients that cannot tolerateadditional stresses. Even though a congestive heart failure may haveadequate ventricular coordination and cardiac output in the presence ofa normal sinus rhythm, when atrial tachyarrhythmia is present,ventricular incoordination may occur, seriously worsening cardiacfunction.

Moreover, some devices treating congestive heart failure sense atrialheart rate and provide biventricular coordination therapy at aventricular heart rate that tracks the atrial heart rate. See, e.g.,Mower U.S. Pat. No. 4,928,688. Such atrial-tracking devices require anormal sinus rhythm to ensure proper delivery of biventricularcoordination therapy. In the presence of atrial tachyarrhythmias, suchas atrial fibrillation, however, such atrial tracking biventricularcoordination therapy could lead to too-fast and irregular biventricularcoordination that is ineffective and even dangerous.

Another problem is that atrial fibrillation may induce irregularventricular heart rhythms by processes that are yet to be fullyunderstood. Such induced ventricular arrhythmias compromise pumpingefficiency even more drastically than atrial arrhythmias. Some devicestreating congestive heart failure provide biventricular coordinationtherapy that does not track the atrial heart rate, but instead, a sensedventricular contraction in a first ventricle triggers a ventricular pacein the other ventricle, or in both ventricles. See, e.g. Mower U.S. Pat.No. 4,928,688. Even if such biventricular coordination therapy isventricular-triggered rather than atrial-tracking, the presence ofatrial tachyarrhythmias could lead to ventricular arrhythmias, such thatthe biventricular coordination therapy becomes ineffective and evendangerous because it is too-fast or irregular because of the irregularventricular heart rate. For these and other reasons, there is a need toprovide congestive heart failure patients with improved pacing andcoordination therapies for improving the AV delay, coordinatingventricular contractions, or otherwise increasing heart pumpingefficiency, even during atrial arrhythmias such as atrial fibrillation.

SUMMARY OF THE INVENTION

The present system provides, among other things, a cardiac rhythmmanagement system including techniques for computing an indicated pacinginterval, AV delay, or other timing interval. In one embodiment, avariable indicated pacing interval is computed based at least in part onan underlying intrinsic heart rate. The indicated pacing interval isused to time the delivery of biventricular coordination therapy evenwhen ventricular heart rates are irregular, such as in the presence ofatrial fibrillation. In another embodiment, a variable filter indicatedAV interval is computed based at least in part on an underlyingintrinsic AV interval. The indicated AV interval is used to time thedelivery of atrial tracking biventricular coordination therapy whenatrial heart rhythms are not arrhythmic. Other indicated timingintervals may be similarly determined. The indicated pacing interval, AVdelay, or other timing interval can also be used in combination with asensor indicated rate indicator.

In one embodiment, the system includes a first method. Actual timingintervals between cardiac events are obtained. The system computes afirst indicated timing interval based at least on a most recent actualtiming interval duration and a previous value of the first indicatedtiming interval. The system provides pacing therapy based on the firstindicated timing interval.

In another embodiment, the system includes a second method. The systemobtains atrio-ventricular (AV) intervals between atrial events andsuccessive ventricular events. The system computes a first indicated AVinterval based at least on a most recent AV interval duration and aprevious value of the first indicated AV interval. The system providespacing therapy based on the first indicated AV interval.

In another embodiment, the system includes a third method. The systemobtains V-V intervals between ventricular beats. The system computes afirst indicated pacing interval based at least on a most recent V-Vinterval duration and a previous value of the first indicated pacinginterval. The system provides pacing therapy to first and secondventricles, based on the first indicated pacing interval.

Another embodiment provides a cardiac rhythm management system thatincludes a sensing circuit for sensing events, a controller obtainingtiming intervals between events and computing a first indicated timinginterval based at least on a most recent actual timing interval durationand a previous value of the first indicated timing interval, and atherapy circuit, providing pacing therapy based on the first indicatedtiming interval.

Another embodiment provides a cardiac rhythm management system thatincludes at least one ventricular sensing circuit, a controller, and aventricular therapy circuit. The controller includes a V-V intervaltimer obtaining V-V intervals between successive events in at least oneventricle, a first register for storing a first indicated pacinginterval, and a filter updating the first indicated pacing intervalbased on a most recent V-V interval provided by the VV interval timerand previous value of the first indicated pacing interval stored thefirst register. The ventricular therapy circuit provides pacing therapyto first and second ventricles based at least partially on the firstindicated pacing interval. Other aspects of the invention will beapparent on reading the following detailed description of the inventionand viewing the drawings that form a part thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

FIG. 1 is a schematic drawing illustrating generally one embodiment ofportions of a cardiac rhythm management system and an environment inwhich it is used.

FIG. 2 is a schematic drawing illustrating one embodiment of a cardiacrhythm management device coupled by leads to a heart.

FIG. 3 is a schematic diagram illustrating generally one embodiment ofportions of a cardiac rhythm management device coupled to a heart.

FIG. 4 is a schematic diagram illustrating generally one embodiment of acontroller.

FIG. 5 is a schematic diagram illustrating generally oneconceptualization of portions of a controller.

FIG. 6 is a signal flow diagram illustrating generally one conceptualembodiment of operating a filter.

FIG. 7 is a signal flow diagram illustrating generally anotherconceptualization of operating the filter.

FIG. 8 is a signal flow diagram illustrating generally a furtherconceptualization of operating the filter.

FIG. 9 is a schematic diagram illustrating generally anotherconceptualization of portions of a controller.

FIG. 10 is a schematic diagram illustrating generally a furtherconceptualization of portions of a controller.

FIG. 11 is a graph illustrating generally one embodiment of operating afilter to provide a first indicated pacing rate, such as a VRR indicatedrate, for successive ventricular heart beats.

FIG. 12 is a graph illustrating generally another embodiment ofoperating a filter to provide the first indicated pacing rate, such as aVRR indicated rate, and delivering therapy based on the first indicatedpacing rate and based on a second indicated pacing rate, such as asensor indicated rate.

FIG. 13 is a graph illustrating generally another illustrative exampleof heart rate vs. time according to a VRR algorithm spreadsheetsimulation.

FIG. 14 is a graph illustrating generally one embodiment of using atleast one of coefficients a and b as a function of heart rate (or acorresponding time interval).

FIG. 15 is a schematic drawing, similar to FIG. 2, illustratinggenerally one embodiment of a cardiac rhythm management device coupledby leads to a heart, such as for providing biventricular coordinationtherapy.

FIG. 16 is a schematic drawing, similar to FIG. 3, illustratinggenerally one embodiment of portions of a cardiac rhythm managementdevice including, among other things, left ventricular sensing andtherapy circuits.

FIG. 17 is a schematic drawing, similar to FIG. 5, illustratinggenerally portions of one conceptual embodiment of a controller.

FIG. 18 is a schematic diagram, similar to FIG. 17, illustratinggenerally another conceptualization of portions of a controller used forregulating an AV interval based on a filter indicated AV delay.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims and their equivalents. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. Like numerals having different lettersuffixes represent different instances of substantially similarcomponents.

The present methods and apparatus will be described in applicationsinvolving implantable medical devices including, but not limited to,implantable cardiac rhythm management systems such as pacemakers,cardioverter/defibrillators, pacer/defibrillators, and biventricular orother multi-site coordination devices. However, it is understood thatthe present methods and apparatus may be employed in unimplanteddevices, including, but not limited to, external pacemakers,cardioverter/defibrillators, pacer/defibrillators, biventricular orother multi-site coordination devices, monitors, programmers andrecorders.

Problems Associated with Atrial Arrhythmias

As stated earlier, one potential cause of irregularity of ventricularcontractions arises during atrial tachyarrhythmias, such as atrialfibrillation. During atrial fibrillation, irregular ventricularcontractions may be caused by an atrial tachyarrhythmia that isconducted to the ventricles. Pacing the ventricles regularizes theventricular heart rate by establishing retrograde conduction from theventricles. This, in turn, is believed to block forward conduction ofatrial signals through the atrioventricular (A-V) node. As a result,irregular atrial signals do not trigger resulting irregular ventricularcontractions.

One therapy for treating irregular ventricular contractions duringatrial fibrillation is to increase the ventricular heart rate by pacingthe ventricles at a higher rate than the average unpaced (intrinsic)ventricular heart rate. Such therapy improves cardiac output because itstabilizes the rate of ventricular contractions to avoid short periodsbetween contractions and/or long periods without a contraction. Suchtherapy is also believed to decrease the ability of the atrialfibrillation to induce irregular ventricular contractions. Additionally,pacing the ventricles at above the average intrinsic ventricular heartrate can provide coordination therapy. Coordination therapy applies thepacing stimulation to one ventricle at multiple sites, to bothventricles at a single site in each ventricle, or to both ventricles atmultiple sites in each ventricle. Coordination therapy is applied to thesites in a fashion that coordinates the sequence of contraction inventricular heart tissue. Coordination therapy is believed to increasesystolic pulse pressure in patients with ventricular conductiondisorders, such as left bundle branch block (LBBB), associated withuncoordinated ventricular contractions. Coordination therapy alsodecreases the time required for systolic contraction, leaving more timefor diastolic ventricular filling, thereby also improving the enddiastolic pressure.

Ventricular Rate Regularization (VRR) Example

This document describes, among other things, a cardiac rhythm managementsystem providing a method and apparatus for treating irregularventricular contractions during atrial arrhythmia by activelystabilizing the ventricular heart rate to obtain less potentiallyproarrhythmic conditions for delivering the atrial tachyarrhythmiatherapy. One suitable technique for stabilizing ventricular heart rateis referred to as Ventricular Rate Regularization, described in Krig etal. U.S. patent application Ser. No. 09/316,515 entitled “Method andApparatus for Treating Irregular Ventricular Contractions Such As DuringAtrial Arrhythmia,” which is filed on even date herewith, assigned tothe assignee of the present patent application, and which is hereinincorporated by reference in its entirety.

FIG. 1 is a schematic drawing illustrating, by way of example, but notby way of limitation, one embodiment of portions of a cardiac rhythmmanagement system 100 and an environment in which it is used. In FIG. 1,system 100 includes an implantable cardiac rhythm management device 105,also referred to as an electronics unit, which is coupled by anintravascular endocardial lead 110, or other lead, to a heart 115 ofpatient 120. System 100 also includes an external programmer 125providing wireless communication with device 105 using a telemetrydevice 130. Catheter lead 110 includes a proximal end 135, which iscoupled to device 105, and a distal end 140, which is coupled to one ormore portions of heart 115.

FIG. 2 is a schematic drawing illustrating, by way of example, but notby way of limitation, one embodiment of device 105 coupled by leads110A-B to heart 115, which includes a right atrium 200A, a left atrium200B, a right ventricle 205A, a left ventricle 205B, and a coronarysinus 220 extending from right atrium 200A. In this embodiment, atriallead 110A includes electrodes (electrical contacts) disposed in, around,or near an atrium 200 of heart 115, such as ring electrode 225 and tipelectrode 230, for sensing signals and/or delivering pacing therapy tothe atrium 200. Lead 110A optionally also includes additionalelectrodes, such as for delivering atrial and/or ventricularcardioversion/defibrillation and/or pacing therapy to heart 115.

In FIG. 2, a ventricular lead 110B includes one or more electrodes, suchas tip electrode 235 and ring electrode 240, for delivering sensingsignals and/or delivering pacing therapy. Lead 110B optionally alsoincludes additional electrodes, such as for delivering atrial and/orventricular cardioversion/defibrillation and/or pacing therapy to heart115. Device 105 includes components that are enclosed in ahermetically-sealed can 250. Additional electrodes may be located on thecan 250, or on an insulating header 255, or on other portions of device105, for providing unipolar pacing and/or defibrillation energy inconjunction with the electrodes disposed on or around heart 115. Otherforms of electrodes include meshes and patches which may be applied toportions of heart 115 or which may be implanted in other areas of thebody to help “steer” electrical currents produced by device 105. Thepresent method and apparatus will work in a variety of configurationsand with a variety of electrical contacts or “electrodes.”

Example Cardiac Rhythm Management Device

FIG. 3 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of portions of device 105,which is coupled to heart 115. Device 105 includes a power source 300,an atrial sensing circuit 305, a ventricular sensing circuit 310, aventricular therapy circuit 320, and a controller 325.

Atrial sensing circuit 305 is coupled by atrial lead 110A to heart 115for receiving, sensing, and/or detecting electrical atrial heartsignals. Such atrial heart signals include atrial activations (alsoreferred to as atrial depolarizations or P-waves), which correspond toatrial contractions. Such atrial heart signals include normal atrialrhythms, and abnormal atrial rhythms including atrial tachyarrhythmias,such as atrial fibrillation, and other atrial activity. Atrial sensingcircuit 305 provides one or more signals to controller 325, via node/bus327, based on the received atrial heart signals. Such signals providedto controller 325 indicate, among other things, the presence of atrialfibrillation.

Ventricular sensing circuit 310 is coupled by ventricular lead 110B toheart 115 for receiving, sensing, and/or detecting electricalventricular heart signals, such as ventricular activations (alsoreferred to as ventricular depolarizations or R-waves), which correspondto ventricular contractions. Such ventricular heart signals includenormal ventricular rhythms, and abnormal ventricular rhythms, includingventricular tachyarrhythmias, such as ventricular fibrillation, andother ventricular activity, such as irregular ventricular contractionsresulting from conducted signals from atrial fibrillation. Ventricularsensing circuit 310 provides one or more signals to controller 325, vianode/bus 327, based on the received ventricular heart signals. Suchsignals provided to controller 325 indicate, among other things, thepresence of ventricular depolarizations, whether regular or irregular inrhythm.

Ventricular therapy circuit 320 provides ventricular pacing therapy, asappropriate, to electrodes located at or near one of the ventricles 205of heart 115 for obtaining resulting evoked ventricular depolarizations.In one embodiment, ventricular therapy circuit 320 also providescardioversion/defibrillation therapy, as appropriate, to electrodeslocated at or near one of the ventricles 205 of heart 115, forterminating ventricular fibrillation and/or other ventriculartachyarrhythmias.

Controller 325 controls the delivery of therapy by ventricular therapycircuit 320 and/or other circuits, based on heart activity signalsreceived from atrial sensing circuit 305 and ventricular sensing circuit310, as discussed below. Controller 325 includes various modules, whichare implemented either in hardware or as one or more sequences of stepscarried out on a microprocessor or other controller. Such modules areillustrated separately for conceptual clarity; it is understood that thevarious modules of controller 325 need not be separately embodied, butmay be combined and/or otherwise implemented, such as insoftware/firmware.

In general terms, sensing circuits 305 and 310 sense electrical signalsfrom heart tissue in contact with the catheter leads 110A-B to whichthese sensing circuits 305 and 310 are coupled. Sensing circuits 305 and310 and/or controller 325 process these sensed signals. Based on thesesensed signals, controller 325 issues control signals to therapycircuits, such as ventricular therapy circuit 320, if necessary, for thedelivery of electrical energy (e.g., pacing and/or defibrillationpulses) to the appropriate electrodes of leads 110A-B. Controller 325may include a microprocessor or other controller for execution ofsoftware and/or firmware instructions. The software of controller 325may be modified (e.g., by remote external programmer 105) to providedifferent parameters, modes, and/or functions for the implantable device105 or to adapt or improve performance of device 105.

In one further embodiment, one or more sensors, such as sensor 330, mayserve as inputs to controller 325 for adjusting the rate at which pacingor other therapy is delivered to heart 115. One such sensor 330 includesan accelerometer that provides an input to controller 325 indicatingincreases and decreases in physical activity, for which controller 325increases and decreases pacing rate, respectively. Another such sensorincludes an impedance measurement, obtained from body electrodes, whichprovides an indication of increases and decreases in the patient'srespiration, for example, for which controller 325 increases anddecreases pacing rate, respectively. Any other sensor 330 providing anindicated pacing rate can be used.

FIG. 4 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one embodiment of controller 325 thatincludes several different inputs to modify the rate at which pacing orother therapy is delivered. For example, Input #1 may provideinformation about left ventricular rate, Input #2 may provide anaccelerometer-based indication of activity, and Input #3 may provide animpedance-based indication of respiration, such as minute ventilation.Based on at least one of these and/or other inputs, controller 325provides an output indication of pacing rate as a control signaldelivered to a therapy circuit, such as to ventricular therapy circuit320. Ventricular therapy circuit 320 issues pacing pulses based on oneor more such control signals received from controller 325. Control ofthe pacing rate may be performed by controller 325, either alone or incombination with peripheral circuits or modules, using software,hardware, firmware, or any combination of the like. The softwareembodiments provide flexibility in how inputs are processed and may alsoprovide the opportunity to remotely upgrade the device software whilestill implanted in the patient without having to perform surgery toremove and/or replace the device 105.

Controller Example 1

FIG. 5 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, one conceptualization of portions ofcontroller 325. At least one signal from ventricular sensing circuit 310is received by ventricular event module 500, which recognizes theoccurrence of ventricular events included within the signal. Such eventsare also referred to as “beats,” “activations,” “depolarizations,” “QRScomplexes,” “R-waves,” “contractions.” Ventricular event module 500detects intrinsic events (also referred to as sensed events) from thesignal obtained from ventricular sensing circuit 310. Ventricular eventmodule 500 also detects evoked events (resulting from a pace) eitherfrom the signal obtained from ventricular sensing circuit 310, orpreferably from a ventricular pacing control signal obtained from pacingcontrol module 505, which also triggers the delivery of a pacingstimulus by ventricular therapy circuit 320. Thus, ventricular eventsinclude both intrinsic/sensed events and evoked/paced events.

A time interval between successive ventricular events, referred to as aV-V interval, is recorded by a first timer, such as V-V interval timer510. A filter 515 computes a “first indicated pacing interval,” i.e.,one indication of a desired time interval between ventricular events or,stated differently, a desired ventricular heart rate. The firstindicated pacing interval is also referred to as a ventricular rateregularization (VRR) indicated pacing interval. In various embodiments,filter 515 includes an averager, a weighted averager, a median filter,an infinite (IIR) filter, a finite impulse response (FIR) filter, or anyother analog or digital signal processing circuit providing the desiredsignal processing described more particularly below.

In one embodiment, filter 515 computes a new value of the firstindicated pacing interval based on the duration of the most recent V-Vinterval recorded by timer 510 and on a previous value of the firstindicated pacing interval stored in first indicated pacing intervalregister 520. Register 520 is then updated by storing the newly computedfirst indicated pacing interval in register 520. Based on the firstindicated pacing interval stored in register 520, pacing control module505 delivers control signals to ventricular therapy circuit 320 fordelivering therapy, such as pacing stimuli, at the VRR-indicatedventricular heart rate corresponding to the inverse of the duration ofthe first indicated pacing interval.

Filter Example 1

In general terms, for one embodiment, device 105 obtains V-V intervalsbetween successive sensed or evoked ventricular beats. Device 105computes a new first indicated pacing interval based at least in part onthe duration of the most recent V-V interval and a previous value of thefirst indicated pacing interval. Device 105 provides pacing therapydelivered at a rate corresponding to the inverse of the duration of thefirst indicated pacing interval.

FIG. 6 is a signal flow diagram illustrating generally, by way ofexample, but not by way of limitation, one embodiment of operatingfilter 515. Upon the occurrence of a sensed or evoked ventricular beat,timer 510 provides filter 515 with the duration of the V-V intervalconcluded by that beat, which is referred to as the most recent V-Vinterval (VV_(n)). Filter 515 also receives the previous value of thefirst indicated pacing interval (T_(n−1)) stored in register 520. Themost recent V-V interval VV_(n) and the previous value of the firstindicated pacing interval T_(n−1) are each scaled by respectiveconstants A and B, and then summed to obtain a new value of the firstindicated pacing interval (T_(n)), which is stored in register 520 andprovided to pacing control module 505. In one embodiment, thecoefficients A and B are different values, and are either programmable,variable, or constant.

If no ventricular beat is sensed during the new first indicated pacinginterval T_(n), which is measured as the time from the occurrence of theventricular beat concluding the most recent V-V interval VV_(n), thenpacing control module 505 instructs ventricular therapy circuit 320 todeliver a ventricular pacing pulse upon the expiration of the new firstindicated pacing interval T_(n). In one embodiment, operation of thefilter is described by T_(n)=A·VV_(n)+B·T_(n−1), where A and B arecoefficients (also referred to as “weights”), VV_(n) is the most recentV-V interval duration, and T_(n−1) is the previous value of the firstindicated pacing interval.

Initialization of filter 515 includes seeding the filter by storing, inregister 520, an initial interval value. In one embodiment, register 520is initialized to an interval value corresponding to a lower rate limit(LRL), i.e., a minimum rate at which pacing pulses are delivered bydevice 105. Register 520 could alternatively be initialized with anyother suitable value.

Filter Example 2

In one embodiment, operation of filter 515 is based on whether the beatconcluding the most recent V-V interval VV_(n) is a sensed/intrinsicbeat or a paced/evoked beat. In this embodiment, the pacing controlmodule 505, which controls the timing and delivery of pacing pulses,provides an input to filter 515 that indicates whether the most recentV-V interval VV_(n) was concluded by an evoked beat initiated by apacing stimulus delivered by device 105, or was concluded by anintrinsic beat sensed by ventricular sensing circuit 310.

In general terms, if the most recent V-V interval VV_(n) is concluded bya sensed/intrinsic beat, then filter 515 provides a new first indicatedpacing interval T_(n) that is adjusted from the value of the previousfirst indicated pacing interval T_(n−1) such as, for example, decreasedby an amount that is based at least partially on the duration of themost recent V-V interval VV_(n) and on the duration of the previousvalue of the first indicated pacing interval T_(n−1). If, however, themost recent V-V interval VV_(n) is concluded by a paced/evoked beat,then filter 515 provides a new first indicated pacing interval T_(n)that is increased from the value of the previous first indicated pacinginterval T_(n−1), such as, for example, by an amount that is based atleast partially on the duration of the most recent V-V interval VV_(n)and on the duration of the previous value of the first indicated pacinginterval T_(n−1). If no ventricular beat is sensed during the new firstindicated pacing interval T_(n), which is measured as the time from theoccurrence of the ventricular beat concluding the most recent V-Vinterval VV_(n), then pacing control module 505 instructs ventriculartherapy circuit 320 to deliver a ventricular pacing pulse upon theexpiration of the new first indicated pacing interval T_(n).

FIG. 7 is a signal flow diagram, illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofoperating filter 515, with certain differences from FIG. 6 moreparticularly described below. In this embodiment, the pacing controlmodule 505, which controls the timing and delivery of pacing pulses,provides an input to filter 515 that indicates whether the most recentV-V interval VV_(n) was concluded by an evoked beat initiated by apacing stimulus delivered by device 105, or was concluded by anintrinsic beat sensed by ventricular sensing circuit 310.

If the most recent V-V interval VV_(n) was concluded by an intrinsicbeat, then the most recent V-V interval VV_(n) and the previous value ofthe first indicated pacing interval T_(n−1) are each scaled byrespective constants A and B, and then summed to obtain the new value ofthe first indicated pacing interval T_(n), which is stored in register520 and provided to pacing control module 505. Alternatively, if themost recent V-V interval VV_(n) was concluded by a evoked/paced beat,then the most recent V-V interval VV_(n) and the previous value of thefirst indicated pacing interval T_(n−1) are each scaled by respectiveconstants C and D, and then summed to obtain the new value of the firstindicated pacing interval T_(n), which is stored in register 520 andprovided to pacing control module 505. In one embodiment, thecoefficients C and D are different from each other, and are eitherprogrammable, variable, or constant. In a further embodiment, thecoefficient C is a different value from the coefficient A, and/or thecoefficient D is a different value than the coefficient B, and thesecoefficients are either programmable, variable, or constant. In anotherembodiment, the coefficient D is the same value as the coefficient B.

In one embodiment, operation of filter 515 is described byT_(n)=A·VV_(n)+B·T_(n−1), if VV_(n) is concluded by an intrinsic beat,and is described by T_(n)=C·VV_(n)+D·T_(n−1), if VV_(n) is concluded bya paced beat, where A, B, C and D are coefficients (also referred to as“weights”), VV_(n) is the most recent V-V interval duration, T_(n) isthe new value of the first indicated pacing interval, and T_(n−1) is theprevious value of the first indicated pacing interval. If no ventricularbeat is sensed during the new first indicated pacing interval T_(n),which is measured as the time from the occurrence of the ventricularbeat concluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the new first indicatedpacing interval T_(n).

Filter Example 3

In another embodiment, these coefficients can be more particularlydescribed using an intrinsic coefficient (a), a paced coefficient (b),and a weighting coefficient (w). In one such embodiment, A=a·w, B=(1−w),C=b·w, and D=(1−w). In one example, operation of the filter 515 isdescribed by T_(n)=a·w·VV_(n)+(1−w)·T_(n−1), if VV_(n) is concluded byan intrinsic beat, otherwise is described byT_(n)=b·w·VV_(n)+(1−w)·T_(n−1), if VV_(n) is concluded by a paced beat,as illustrated generally, by way of example, but not by way oflimitation, in the signal flow graph of FIG. 8. If no ventricular beatis sensed during the new first indicated pacing interval T_(n), which ismeasured as the time from the occurrence of the ventricular beatconcluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the new first indicatedpacing interval T_(n). In one embodiment, the coefficients a and b aredifferent from each other, and are either programmable, variable, orconstant.

The above-described parameters (e.g., A, B, C, D, a, b, w) are stated interms of time intervals (e.g., VV_(n), T_(n), T_(n−1)). However, analternate system may produce results in terms of rate, rather than timeintervals, without departing from the present method and apparatus. Inone embodiment, weighting coefficient w, intrinsic coefficient a, andpaced coefficient b, are variables. Different selections of w, a, and b,will result in different operation of the present method and apparatus.For example, as w increases the weighting effect of the most recent V-Vinterval VV_(n) increases and the weighting effect of the previous firstindicated pacing rate T_(n−1) decreases. In one embodiment, w={fraction(1/16)}=0.0625. In another embodiment, w={fraction (1/32)}. Anotherpossible range for w is from w={fraction (1/2)} to w={fraction(1/1024)}. A further possible range for w is from w≈0 to w≈1. Othervalues of w, which need not include division by powers of two, may besubstituted without departing from the present method and apparatus.

In one embodiment, intrinsic coefficient a, is selected to be greaterthan 0.5, or to be greater than 1.0. In one example, the intrinsiccoefficient a is selected to be lesser in value than the pacingcoefficient b. In one example, a≈1.1 and b≈1.2. In another embodimenta=0.9 and b=1.1. One possible range for a is from a=0.5 to a=2.0, andfor b is from b=1.0 to b=3.0. The coefficients may vary withoutdeparting from the present method and apparatus.

In one embodiment, for b>1 and for substantially regular V-V intervals,filter 515 provides a new first indicated pacing interval T_(n) that isat least slightly longer than the expected intrinsic V-V interval beingmeasured by timer 515. Thus, if the intrinsic V-V interval being timedis consistent with the duration of previously received V-V intervals,then filter 515 avoids triggering a pacing stimulus. In such a case, apacing pulse is delivered only if the presently timed V-V intervalbecomes longer than the previous substantially constant V-V intervals.In general terms, filter 515 operates so that pacing pulses aretypically inhibited if the ventricular rate is substantially constant.However, if the measured V-V intervals become irregular, then filter 515operates, over a period of one or several such V-V intervals, to shortenthe first indicated pacing interval T_(n) so that pacing stimuli arebeing delivered.

According to one aspect of the invention, it is believed that if theirregular V-V intervals are caused by a conducted atrialtachyarrhythmia, then pacing the ventricle will regularize theventricular heart rate by establishing retrograde conduction from theventricle. This, in turn, blocks forward conduction of atrial signalsthrough the atrioventricular (A-V) node. As a result, irregular atrialsignals do not trigger resulting irregular ventricular contractions.According to another aspect of the invention, however, this method andapparatus will not introduce pacing pulses until the heartbeat becomesirregular. Therefore, the heart is assured to pace at its intrinsic ratewhen regular ventricular contractions are sensed.

Controller Example 2

FIG. 9 is a schematic diagram illustrating generally, by way of example,but not by way of limitation, another conceptualization of portions ofcontroller 325, with certain differences from FIG. 5 more particularlydescribed below. In FIG. 9, controller 325 receives from sensor 330 asignal including information from which a physiologically desired heartrate (e.g., based on the patient's activity, respiration, or any othersuitable indicator of metabolic need) can be derived. The sensor signalis digitized by an A/D converter 900. The digitized signal is processedby a sensor rate module 905, which computes a desired heart rate that isexpressed in terms of a second indicated pacing interval stored inregister 910.

Pacing control module 505 delivers a control signal, which directsventricular therapy circuit 320 to deliver a pacing pulse, based oneither (or both) of the first or second indicated pacing intervals,stored in registers 520 and 910, respectively, or both. In oneembodiment, pacing control module 505 includes a selection module 915that selects between the new first indicated pacing interval T_(n) andthe sensor-based second indicated pacing interval.

In one embodiment, selection module 915 selects the shorter of the firstand second indicated pacing intervals as the selected indicated pacinginterval S_(n). If no ventricular beat is sensed during the selectedindicated pacing interval S_(n), which is measured as the time from theoccurrence of the ventricular beat concluding the most recent V-Vinterval VV_(n), then pacing control module 505 instructs ventriculartherapy circuit 320 to deliver a ventricular pacing pulse upon theexpiration of the selected indicated pacing interval S_(n).

In general terms, for this embodiment, the ventricle is paced at thehigher of the sensor indicated rate and the VRR indicated rate. If, forexample, the patient is resting, such that the sensor indicated rate islower than the patient's intrinsic rate, and the patient's intrinsicrate is substantially constant, then the intrinsic rate is higher thanthe VRR indicated rate. As a result, pacing pulses generally will not bedelivered. But if, for example, the patient is resting, but with anatrial tachyarrhythmia that induces irregular ventricular contractions,then pacing pulses generally will be delivered at the VRR indicatedrate. In another example, if the patient is active, such that the sensorindicated rate is higher than the VRR indicated rate, then pacing pulsesgenerally will be delivered at the sensor indicated rate. In analternative embodiment, the pacing rate is determined by blending thesensor indicated rate and the VRR indicated rate, rather than byselecting the higher of these two indicated rates (i.e., the shorter ofthe first and second indicated pacing intervals).

In another embodiment, selection module 915 provides a selectedindicated pacing interval S_(n) based on a blending of both the firstand second indicated pacing intervals. In one such example, selectionmodule 915 applies predetermined or other weights to the first andsecond indicated pacing intervals to compute the selected pacinginterval S_(n).

Controller Example 2

FIG. 10 is a schematic diagram illustrating generally, by way ofexample, but not by way of limitation, another conceptualization ofportions of controller 325, with certain differences from FIG. 9 moreparticularly described below. In FIG. 10, controller 325 includes anatrial tachyarrhythmia (AT) detection module 1000 that receives a signalfrom atrial sensing circuit 305. The received signal includesinformation about atrial events, from which AT detection module 1000determines the presence or absence of one or more atrialtachyarrhythmias, such as atrial fibrillation.

In one embodiment, AT detection module 1000 provides a control signal,to pacing control module 505, that indicates the presence or absence ofan atrial tachyarrhythmia, such as atrial fibrillation. In oneembodiment, selection module 915 selects between the first and secondindicated pacing intervals as illustrated, by way of example, but not byway of limitation, in Table 1.

TABLE 1 Example Selection Based on AT Detection, 1st Indicated PacingInterval, and 2nd Indicated Pacing Interval 1st Indicated Pacing 1stIndicated Pacing Interval < 2nd Indicated Interval ≧ 2nd Indicated ATPresent? Pacing Interval? Pacing Interval? Yes, AT Present S_(n) ← 1stIndicated S_(n) ← 2nd Indicated Pacing Interval (i.e., Pacing Interval(e.g., VRR) Sensor) No, AT not Present S_(n) ← 2nd Indicated S_(n) ← 2ndIndicated Pacing Interval (e.g., Pacing Interval (e.g., Sensor) Sensor)

In this embodiment, if an atrial tachyarrhythmia is present and thefirst indicated pacing interval is shorter than the second indicatedpacing interval, then selection module 915 selects the first indicatedpacing interval, which is based on the VRR techniques described above,as the selected indicated pacing interval S_(n). Otherwise, selectionmodule 915 selects the second indicated pacing interval, which in oneembodiment is based on the sensor indications, as the selected indicatedpacing interval S_(n). As discussed above, if no ventricular beat issensed during the selected indicated pacing interval S_(n), which ismeasured as the time from the occurrence of the ventricular beatconcluding the most recent V-V interval VV_(n), then pacing controlmodule 505 instructs ventricular therapy circuit 320 to deliver aventricular pacing pulse upon the expiration of the selected indicatedpacing interval S_(n).

Stated differently, for this embodiment, the ventricle is paced at theVRR indicated rate only if an atrial tachyarrhythmia, such as atrialfibrillation, is present and the VRR indicated rate exceeds the sensorindicated rate. Otherwise the ventricle is paced at the sensor indicatedrate. If, for example, the patient is resting, such that the sensorindicated rate is lower than the patient's intrinsic rate, and no atrialtachyarrhythmia is present, then the device will sense the intrinsicrate or will deliver ventricular paces at the lower rate limit. But if,for example, the patient is resting, but with an atrial tachyarrhythmiathat induces irregular ventricular contractions, then pacing pulsesgenerally will be delivered at the VRR indicated rate. In anotherexample, if the patient is active, such that the sensor indicated rateis higher than the VRR indicated rate, then pacing pulses generally willbe delivered at the sensor indicated rate, whether or not atrialtachyarrhythmia is present. As an alternative to the selection describedwith respect to Table 1, selection module 915 provides a fixed orvariable weighting or blending of both the sensor-indicated rate and VRRindicated rate, such that pacing pulses are delivered based on theblended rate.

The second indicated pacing interval need not be based on sensorindications. In one embodiment, for example, the second indicated pacinginterval tracks the sensed atrial heart rate when no atrialtachyarrhythmia is present. In this embodiment, selection module 915performs a mode-switching function in which the first indicated pacinginterval is used whenever atrial tachyarrhythmia is present and thesecond indicated pacing interval (e.g., atrial-tracking) is used when noatrial tachyarrhythmia is present.

In another embodiment, heart rate/interval is used as a trigger to turnon/off use of the first indicated pacing interval (e.g., the VRRindicated pacing interval). In one example, pacing therapy is based onthe first indicated pacing interval if the first indicated pacinginterval is longer than a first predetermined value, and pacing therapyis substantially independent of the first indicated pacing interval ifthe first indicated pacing interval is shorter than the firstpredetermined value. In this example, the VRR indicated pacing intervalis used at low heart rates, but not at fast heart rates.

Filter Rate Behavior Example 1

FIG. 11 is a graph illustrating generally, by way of example, but not byway of limitation, one embodiment of a VRR indicated rate for successiveventricular heart beats for one mode of operating filter 515. Asdiscussed above, the VRR indicated rate is simply the frequency, betweenventricular heart beats, associated with the first indicated pacinginterval. Stated differently, the VRR indicated rate is the inverse ofthe duration of the first indicated pacing interval. If pacing is basedsolely on the VRR indicated rate, pacing control module 505 directsventricular therapy circuit 320 to issue a pacing pulse after the timesince the last ventricular beat equals or exceeds the first indicatedpacing interval. However, as described above, in certain embodiments,pacing control module 505 directs ventricular therapy circuit 320 toissue a pacing pulse based on factors other than the VRR indicated ratesuch as for, example, based on the sensor indicated rate.

In the example illustrated in FIG. 11, a first sensed intrinsicventricular beat, indicated by an “S” was detected just beforeexpiration of the first indicated pacing interval (“VRR indicated pacinginterval”) T₀, as computed based on a previous ventricular beat. In oneembodiment, the new VRR indicated pacing interval T₁ is computed basedon the duration of most recent V-V interval VV₁ and a previous value ofthe VRR indicated pacing interval T₀, as discussed above. In thisexample, the new VRR indicated pacing interval T₁ corresponds to a lowerrate limit (LRL) time interval. In one embodiment, the allowable rangeof the VRR indicated pacing interval is limited so that the VRRindicated pacing interval does not exceed the duration of the LRL timeinterval, and so that the VRR indicated pacing interval is not shorterthan the duration of an upper rate limit (URL) time interval.

The second ventricular beat is also sensed, just before expiration ofthe VRR indicated pacing interval T₁. In one embodiment, the new VRRindicated pacing interval T₂ is computed based on the duration of mostrecent V-V interval VV₂ and a previous value of the VRR indicated pacinginterval, T₁, as discussed above. The first and second ventricular beatsrepresent a stable intrinsic rhythm, for which no pacing is deliveredbecause the VRR indicated pacing interval is at a lower rate than thesensed intrinsic ventricular beats.

The third, fourth, and fifth ventricular beats represent the onset ofatrial fibrillation, resulting in erratic ventricular rates. The thirdventricular beat is sensed well before expiration of the VRR indicatedpacing interval T₂, such that no pacing pulse is issued. For the sensedthird ventricular beat, filter 515 computes the new VRR indicated pacinginterval T₃ as being shorter in duration relative to the previous VRRindicated pacing interval T₂.

The fourth ventricular beat is similarly sensed well before expirationof the VRR indicated pacing interval T₃, such that no pacing pulse isissued. For the sensed fourth ventricular beat, filter 515 computes thenew VRR indicated pacing interval T₄ as being shorter in durationrelative to the previous VRR indicated pacing interval T₃.

The fifth ventricular beat is sensed before expiration of the VRRindicated pacing interval T₄, such that no pacing pulse is issued. Forthe sensed fifth ventricular beat, filter 515 computes the new VRRindicated pacing interval T₅ as being shorter in duration relative tothe previous VRR indicated pacing interval T₄.

The sixth, seventh, and eighth ventricular beats indicate regularizationof the ventricular rate using the pacing techniques described above. Noventricular beat is sensed during the VRR indicated pacing interval T₅,so a pacing pulse is issued to evoke the sixth ventricular beat. A newVRR indicated pacing interval T₆ is computed as being increased induration relative to the previous VRR indicated pacing interval T₅,lowering the VRR indicated rate. Similarly, no ventricular beat issensed during the VRR indicated pacing interval.

The ninth ventricular beat represents another erratic ventricular beatresulting from the atrial fibrillation episode. The ninth ventricularbeat is sensed before expiration of the VRR indicated pacing intervalT₈. As a result, a shorter new VRR indicated pacing interval T₉ iscomputed.

The tenth and eleventh ventricular beats illustrate furtherregularization of the ventricular rate using the pacing techniquesdescribed above. No ventricular beat is sensed during the VRR indicatedpacing interval T₉, so a pacing pulse is issued to evoke the tenthventricular beat. A new VRR indicated pacing interval T₁₀ is computed asbeing increased in duration relative to the previous VRR indicatedpacing interval T₉, lowering the VRR indicated rate. Similarly, noventricular beat is sensed during the VRR indicated pacing interval T₁₀,so a pacing pulse is issued to evoke the tenth ventricular beat. A newVRR indicated pacing interval T₁₁ is compute as being increased induration relative to the previous VRR indicated pacing interval T₁₀,lowering the VRR indicated rate.

The twelfth, thirteenth, fourteenth, and fifteenth ventricular beatsillustrate resumption of a stable intrinsic rhythm after termination ofthe atrial fibrillation episode. For such a stable rate, the VRRindicated rate proceeds asymptotically toward a “floor value” thattracks, but remains below, the intrinsic rate. This allows the intrinsicheart signals to control heart rate when such intrinsic heart signalsprovide a stable rhythm. As a result, when the patient's intrinsic rateis constant, paces will be withheld, allowing the patient's intrinsicheart rhythm to continue. If the patient's heart rate includes somevariability, and the VRR indicated floor value is close to the meanintrinsic heart rate, then occasional paced beats will occur. Such pacebeats will gradually lengthen the VRR indicated pacing interval, therebyallowing subsequent intrinsic behavior when the patient's heart ratebecomes substantially constant.

The intrinsic coefficient a of filter 515 controls the “attack slope” ofthe VRR indicated heart rate as the VRR indicated heart rate increasesbecause of sensed intrinsic beats. The paced coefficient b of filter 515controls the “decay slope” of the VRR indicated heart rate as the VRRindicated heart rate decreases during periods of paced beats. In oneembodiment, in which a>1.0 and b>1.0, decreasing the value of a toward1.0 increases the attack slope such that the VRR indicated rateincreases faster in response to sensed intrinsic beats, while decreasingthe value of b toward 1.0 decreases the decay slope such that the VRRindicated rate decreases more slowly during periods of paced beats.Conversely, for a>1.0 and b>1.0, increasing the value of a from 1.0decreases the attack slope such that the VRR indicated rate increasesmore slowly in response to sensed intrinsic beats, while increasing thevalue of b from 1.0 increases the decay slope such that theVRR-indicated rate decreases more quickly during periods of paced beats.

In one embodiment, for a>1.0 and b>1.0, decreasing both a and b toward1.0 increases VRR indicated rate during periods of sensed intrinsicactivity so that the VRR indicated rate is closer to the mean intrinsicrate. Because the VRR indicated rate is closer to the mean intrinsicrate, variability in the intrinsic heart rate is more likely to triggerpaces at the VRR indicated rate. On the other hand, for a>1.0 and b>1.0,increasing both a and b from 1.0 decreases the VRR indicated rate duringperiods of sensed intrinsic activity so that the VRR indicated rate isfarther beneath the mean intrinsic rate. Because the VRR indicated rateis farther beneath the mean intrinsic rate, the same variability in theintrinsic heart rate becomes less likely to trigger paces at the VRRindicated rate.

In one embodiment, these coefficients are programmable by the user, suchas by using remote programmer 125. In another embodiment, the userselects a desired performance parameter (e.g., desired degree of rateregularization, desired attack slope, desired decay slope, etc.) from acorresponding range of possible values, and device 105 automaticallyselects the appropriate combination of coefficients of filter 515 toprovide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table2. Other levels of programmability or different combinations ofcoefficients may also be used.

TABLE 2 Example of Automatic Selection of Aspects of Filter SettingBased on a User-Programmable Performance Parameter. User-ProgrammablePerformance Parameter Intrinsic Coefficient a Paced Coefficient b 1(Less Rate 2.0 3.0 Regularization) 2 1.8 2.6 3 1.6 2.2 4 1.4 1.8 5 1.21.4 6 (More Rate 1.0 1.0 Regularization)

Filter Rate Behavior Example 2

Figure 12 is a graph illustrating generally, by way of example, but notby way of limitation, one embodiment of selecting between more than oneindicated pacing interval. FIG. 12 is similar to FIG. 11 in somerespects, but FIG. 12 includes a second indicated pacing interval. Inone embodiment, the first indicated pacing interval is the VRR indicatedpacing interval, described above, and the second indicated pacinginterval is a sensor indicated pacing interval, from an accelerometer,minute ventilation, or other indication of the patient's physiologicalneed for increased cardiac output.

In one embodiment, a selected indicated pacing interval is based on theshorter of the first and second indicated pacing intervals. Stateddifferently, device 105 provides pacing pulses at the higher indicatedpacing rate. In the example illustrated in FIG. 12, first and secondbeats and the twelfth through fifteenth beats are paced at the sensorindicated rate, because it is higher than the VRR indicated rate and theintrinsic rate. The third, fourth, fifth, and ninth beats are sensedintrinsic beats that are sensed during the shorter of either of the VRRand sensor indicated pacing intervals. The sixth through eighth beatsand tenth and eleventh beats are paced at the VRR indicated rate,because it is higher than the sensor indicated rate. Also, for thesebeats, no intrinsic beats are sensed during the VRR indicated intervals.In one embodiment, the above-described equations for filter 515 operateto increase the VRR indicated rate toward the sensor-indicated rate whenthe sensor indicated rate is greater than the VRR indicated rate, asillustrated by first through third and twelfth through fifteenth beatsin FIG. 12. In an alternate embodiment, however,T_(n)=b·w·VV_(n)+(1−w)·T_(n−1), if VV_(n) is concluded by a VRRindicated paced beat, and T_(n)=T_(n−1) if VV_(n) is concluded by asensor indicated paced beat, thereby leaving the VRR indicated rateunchanged for sensor indicated paced beats.

In this embodiment, the ranges of both the sensor indicated rate and theVRR indicated rate are limited so that they do not extend to rateshigher than the URL or to rates lower than the LRL. In one embodiment,the LRL and the URL are programmable by the user, such as by usingremote programmer 125.

In a further embodiment, the selected indicated pacing interval is basedon the shorter of the first and second indicated pacing intervals onlyif an atrial tachyarrhythmia, such as atrial fibrillation, is present.Otherwise, the second indicated pacing interval is used, as describedabove.

Filter Rate Behavior Example 3

FIG. 13 is a graph illustrating generally, by way of example, but not byway of limitation, another illustrative example of heart rate vs. timeaccording to a spreadsheet simulation of the behavior of theabove-described VRR algorithm. In FIG. 13, the VRR algorithm is turnedoff until time 130. Stable intrinsic lower rate behavior is modeled fortimes between 0 and 10 seconds. Erratic intrinsic ventricular rates,such as would result from atrial tachyarrhythmias including atrialfibrillation, are modeled during times between 10 seconds and 130seconds. At time 130 seconds, the VRR algorithm is turned on. While someerratic intrinsic beats are subsequently observed, the VRR algorithmprovides pacing that is expected to substantially stabilize the heartrate, as illustrated in FIG. 13. The VRR indicated pacing rate graduallydecreases until intrinsic beats are sensed, which results in a slightincrease in the VRR indicated pacing rate. Thus, the VRR algorithmfavors the patient's intrinsic heart rate when it is stable, and pacesat the VRR indicated heart rate when the patient's intrinsic heart rateis unstable. It is noted that FIG. 13 does not represent clinical data,but rather provides a simulation model that illustrates one example ofhow the VRR algorithm is expected to operate.

Filter Example 4

In one embodiment, filter 515 includes variable coefficients such as,for example, coefficients that are a function of heart rate (or itscorresponding time interval). In one example, operation of the filter515 is described by T_(n)=a·w·VV_(n)+(1−w)·T_(n−1), if VV_(n) isconcluded by an intrinsic beat, otherwise is described byT_(n)=b·w·VV_(n)+(1−w)·T_(n−1), if VV_(n) is concluded by a paced beat,where at least one of a and b are linear, piecewise linear, or nonlinearfunctions of one or more previous V-V intervals such as, for example,the most recent V-V interval, VV_(n).

FIG. 14 is a graph illustrating generally, by way of example, but not byway of limitation, one embodiment of using at least one of coefficientsa and b as a function of one or more previous V-V intervals such as, forexample, the most recent V-V interval, VV_(n). In one such example, a isless than 1.0 when VV_(n) is at or near the lower rate limit (e.g., 1000millisecond interval or 60 beats/minute), and a is greater than 1.0 whenVV_(n) is at or near the upper rate limit (e.g., 500 millisecondinterval or 120 beats/minute). For a constant b, using a smaller valueof a at lower rates will increase the pacing rate more quickly forsensed events; using a larger value of a at higher rates increases thepacing rate more slowly for sensed events. In another example, b isclose to 1.0 when VV_(n) is at or near the lower rate limit, and b isgreater than 1.0 when VV_(n) is at or near the upper rate limit. For aconstant a, using a smaller value of b at lower rates will decrease thepacing rate more slowly for paced events; using a larger value of b athigher rates decreases the pacing rate more quickly for paced events.

Biventricular Coordination Therapy Example

In one embodiment, the present cardiac rhythm management system utilizesthe VRR filter 515 for providing biventricular pacing coordinationtherapy in both right and left ventricles, thereby coordinating thecontractions of the right and left ventricles for more efficientpumping. The VRR filter 515 controls the timing of delivery ofbiventricular pacing pulses. Filter 515 provides a first indicatedpacing rate that is independent of (i.e., does not track) the intrinsicatrial rate. The first indicated pacing rate is generally above the meanintrinsic ventricular rate so that it can provide substantiallycontinuous pacing therapy. This is advantageous because, among otherthings, either or both of the intrinsic atrial or intrinsic ventricularheart rates may be extremely irregular, such as during atrialtachyarrhythmias. The VRR techniques described above with respect toFIGS. 1-14 promote intrinsic ventricular beats when the ventricularheart rate is substantially constant. In contrast to such VentricularRate Regularization that promotes intrinsic activity, biventricularcoordination therapy typically uses Ventricular Rate Regulation thatprovides nearly continuous (i.e., to the desired degree) biventricularpacing when the ventricular rate is substantially constant. Oneselection of coefficients for filter 515 for obtaining nearly continuouspacing is described below.

FIG. 15 is a schematic drawing, similar to FIG. 2, illustratinggenerally by way of example, but not by way of limitation, oneembodiment of a cardiac rhythm management device 105 coupled by leads110A-C to a heart 115. In one such embodiment, system 100 providesbiventricular coordination therapy to coordinate right ventricular andleft ventricular contractions, such as for congestive heart failurepatients. FIG. 15 includes a left ventricular lead 110C, insertedthrough coronary sinus 220 and into the great cardiac vein so that itselectrodes, which include electrodes 1500 and 1505, are associated withleft ventricle 205B for sensing intrinsic heart signals and providingone or more of coordination paces or defibrillation shocks.

FIG. 16 is a schematic drawing, similar to FIG. 3, illustratinggenerally by way of example, but not by way of limitation, oneembodiment of portions of a cardiac rhythm management device 105, inwhich the left ventricular lead is coupled by lead 110C to a leftventricular sensing circuit 1600 and a left ventricular therapy circuit1605, each of which are, in turn, coupled by node/bus 327 to controller325. This embodiment also includes an atrial therapy circuit 1610, and aright ventricular lead 110B coupling right ventricle 205A to rightventricular sensing circuit 310 and right ventricular therapy circuit320, each of which are, in turn, coupled by node/bus 327 to controller325.

FIG. 17 is a schematic drawing, similar to FIG. 5, illustratinggenerally by way of example, but not by way of limitation, portions ofone conceptual embodiment of controller 325. In this embodiment,ventricular event module 500 receives input signals from rightventricular sensing circuit 310 and left ventricular sensing circuit1600. Pacing control module 505 provides control signals to rightventricular therapy circuit 320 and left ventricular therapy circuit1605. Operation of filter 515 is similar to the above descriptionaccompanying FIG. 5-14, with certain differences discussed below,thereby allowing device 105 to provide biventricular coordinationtherapy at a VRR-indicated rate, a sensor-indicated rate, or acombination thereof.

In one embodiment, ventricular event module 500 detects sensed and pacedventricular beats from both right ventricular sensing circuit 310 andleft ventricular sensing circuit 1600. An interval between successiveventricular events, referred to as a V-V interval, is recorded by afirst timer, such as V-V interval timer 510. Ventricular event module500 selects the particular ventricular events initiating and concludingthe V-V interval timed by V-V interval timer 510. In a first mode ofoperation, the V-V interval is initiated by a right ventricular beat(paced or sensed), and the V-V interval is then concluded by the nextright ventricular beat (paced or sensed). In a second mode of operation,the V-V interval is initiated by a left ventricular beat (aced orsensed), and the V-V interval is then concluded by the next leftventricular beat (paced or sensed). In a third mode of operation, theV-V interval is initiated by either a right or left ventricular beat,and the V-V interval is then concluded by the next right or leftventricular beat that occurs after expiration of a refractory period ofapproximately between 130 milliseconds and 500 milliseconds (e.g., 150milliseconds). Left or right ventricular beats occurring during therefractory period are ignored. Using the refractory period ensures thatthe beat concluding the V-V interval is associated with a subsequentventricular contraction, rather than a depolarization associated withthe same ventricular contraction, in which the depolarization is merelysensed in the opposite ventricle from the initiating beat. Such arefractory period can also be used in conjunction with the first mode(V-V interval initiated and concluded by right ventricular beats) or thesecond mode (V-V interval initiated and concluded by left ventricularbeats).

Filter 515 computes a “first indicated pacing interval,” i.e., oneindication of a desired time interval between ventricular events or,stated differently, a desired ventricular heart rate. Based on the firstindicated pacing interval stored in register 520, pacing control module505 delivers control signals to one or more of therapy circuits 320 and340 for delivering therapy, such as biventricular pacing coordinationstimuli to one or more of right ventricle 205A and left ventricle 205B,at the VRR indicated ventricular heart rate corresponding to the inverseof the duration of the first indicated pacing interval.

Ventricular event module 500 also includes an output node/bus 1700coupled to pacing control module 505. Ventricular event module 500communicates to pacing control module 505 information about theoccurrence (e.g., timing, origin, etc.) of right and left ventricularsensed beats, so that pacing control module 505 can issue biventricularcoordination therapy to obtain coordinated right and left ventricularcontractions. In one embodiment, a sensed right ventricular contractiontriggers an immediate or very slightly delayed left ventricular pacingpulse, either alone, or in conjunction with a right ventricular pacingpulse. Similarly, a sensed left ventricular contraction triggers animmediate or very slightly delayed right ventricular pacing pulse,either alone or in conjunction with a left ventricular pacing pulse.This ensures that contractions of the right and left ventricles arecoordinated to provide more efficient pumping of blood by the heart 115.In one embodiment, the controller illustrated in FIG. 17 includes asensor channel, as discussed above with respect to FIG. 9. In thisembodiment, device 105 provides biventricular coordination therapy at aVRR-indicated rate, a sensor-indicated rate, or a combination thereof.

In one embodiment, the coefficients of filter 515 are programmable bythe user, such as by using remote programmer 125, as described above, inorder to obtain a desired degree of pacing vs. sensing. In anotherembodiment, the user selects a desired performance parameter (e.g.,desired degree of pacing vs. sensing, etc.) from a corresponding rangeof possible values, and device 105 automatically selects the appropriatecombination of coefficients of filter 515 to provide a filter settingthat corresponds to the selected user-programmed performance parameter,as illustrated generally by Table 3. Other levels of programmability ordifferent combinations of coefficients may also be used.

TABLE 3 Example of Automatic Selection of Aspects of Filter SettingBased on a User-Programmable Performance Parameter Such as For ProvidingBiventricular Coordination Therapy. User-Programmable PerformanceParameter Intrinsic Coefficient a Paced Coefficient b 1 (More Pacing)0.6  1.05 2 0.7 1.2 3 0.8 1.3 4 0.9 1.4 5 (Less Pacing) 1.0 1.5

In a further embodiment, device 105 uses a mapping, such as illustratedin Table 3, in a feedback control loop to automatically select the“performance parameter” of Table 3 and corresponding coefficients. Theuser programs a mean pacing frequency goal. Device 105 measures the meanpacing frequency over a predetermined period of time or predeterminednumber of V-V intervals. The measured mean pacing is compared to themean pacing frequency goal. If the measured mean pacing frequency ishigher than the goal mean pacing frequency, the performance parameter inTable 3 is incremented/decremented toward less pacing. Conversely, ifthe measured mean pacing frequency is lower than the goal mean pacingfrequency, the performance parameter in Table 3 isincremented/decremented toward more pacing. In a further embodiment, themeasured mean pacing frequency is compared to values that are slightlyoffset about the goal mean pacing frequency (e.g., goal mean pacingfrequency +/−Δ) to provide a band of acceptable measured mean pacingfrequencies within which the performance parameter is not switched.

Av Delay Regulation Embodiment For Congestive Heart Failure Patients

The preceding embodiment illustrated techniques of providing rateregulation for delivering biventricular coordination therapy tocongestive heart failure patients. Such techniques are particularlyadvantageous in the presence of atrial tachyarrhymias, such as atrialfibrillation. When atrial tachyarrhythmias are present, atrial ratetracking would result in too-fast and irregular biventricularcoordination therapy. Because atrial tachyarrhythmias often induceirregular ventricular cardiac cycle lengths, ventricular tracking wouldalso produce erratic results if the above-described VRR techniques arenot used.

Where no atrial tachyarrhythmias are present, however, biventricularcoordination therapy based on atrial rate tracking is possible. Theabove-disclosed techniques are still useful, however, for providing afirst indicated timing interval. In one example, the first indicatedtiming interval is a filter indicated atrioventricular (AV) delay basedon the intrinsic AV delay, i.e., the interval between a sensed P waveand a successively sensed R wave. In this example, biventricularcoordination therapy is provided based on the tracking of an atrialrate. Each biventricular coordination pacing pulse is delivered afterthe filter indicated AV delay. The filter indicated AV delay is computedsimilarly to the VRR techniques described above, with certaindifferences described more particularly below.

FIG. 18 is a schematic diagram, similar to FIG. 17, illustratinggenerally by way of example, but not by way of limitation, anotherconceptualization of portions of controller 325 used for regulating theAV interval based on a filter indicated AV delay. In FIG. 18, atrialevent module 1800 and ventricular event module 500 provide informationabout paced or sensed atrial events and paced or sensed ventricularevents, respectively, to AV interval timer 1805. AV interval timer 1805times an AV interval initiated by an atrial event, and concluded by aventricular event, such as described above with respect to FIG. 17.

In one example, operation of the filter 515 is described byT_(n)=a·w·AV_(n)+(1−w)·T_(n−1), if AV_(n) is concluded by an intrinsicbeat, otherwise is described by T_(n)=b·w·AV_(n)+(1−w)·T_(n−1), ifAV_(n) is concluded by a paced beat, where T_(n) is the newly computedvalue of the filter indicated AV interval, T_(n−1) is the previous valueof the filter indicated AV interval, AV_(n) is the time intervalcorresponding to the most recent AV delay period, and a, b, and w arecoefficients. In one embodiment, weighting coefficient w, intrinsiccoefficient a, and paced coefficient b, are variables. Differentselections of w, a, and b, will result in different operation of thepresent method and apparatus. For example, as w increases the weightingeffect of the most recent A-V interval AV_(n) increases and theweighting effect of the previous first indicated pacing rate T_(n−1)decreases. In one embodiment, w={fraction (1/16)}=0.0625. In anotherembodiment, w={fraction (1/32)}. Another possible range for w is fromw=½ to w={fraction (1/1024)}. A further possible range for w is from w≈0to w≈1. Other values of w, which need not include division by powers oftwo, may be substituted without departing from the present method andapparatus.

In one embodiment, intrinsic coefficient a, is selected to be less than(or, alternatively, less than or equal to) 1.0. In one example, theintrinsic coefficient a is selected to be lesser in value than thepacing coefficient b. In one embodiment, a≈0.6 and b≈1.5. In anotherembodiment, a=1.0 and b=1.05. One possible range for a is from a=0.6 toa=1.0, and for b is from b=1.05 to b=1.5. The coefficients may varywithout departing from the present method and apparatus.

In one embodiment, these coefficients are programmable by the user, suchas by using remote programmer 125. In another embodiment, the userselects a desired performance parameter (e.g., desired degree pacing vs.sensing, desired attack slope, desired decay slope, etc.) from acorresponding range of possible values, and device 105 automaticallyselects the appropriate combination of coefficients of filter 515 toprovide a filter setting that corresponds to the selecteduser-programmed performance parameter, as illustrated generally by Table4. Other levels of programmability or different combinations ofcoefficients may also be used.

TABLE 4 Example of Automatic Selection of Aspects of Filter SettingBased on a User-Programmable Performance Parameter, Such as for AV DelayRegulation User-Programmable Performance Parameter Intrinsic Coefficienta Paced Coefficient b 1 (Less Aggressive 1.0  1.05 Attack/Decay) 2 0.91.2 3 0.8 1.3 4 0.7 1.4 5 (More Aggressive 0.6 1.5 Attack/Decay)

In a further embodiment, device 105 uses a mapping, such as illustratedin Table 4, in a feedback control loop to automatically select the“performance parameter” and corresponding coefficients. The userprograms a mean sense frequency goal. Device 105 measures the meanfrequency of sensed ventricular events (“measured mean sense frequency”)over a predetermined period of time or predetermined number of A-Vintervals, and adjusts the performance parameter and correspondingcoefficients to direct the measured mean sense frequency toward the meansense frequency goal.

The above techniques provide an example in which AV delay is regulated.However, it is understood that these techniques extend to the regulationof any other timing interval. In one embodiment, for example, operationof filter 515 is expressed more generically as T_(n)=A·EE_(n)+B·T_(n−1),if EE_(n) is concluded by an intrinsic beat, otherwise is described byT_(n)=C·EE_(n)+D·T_(n−1), if EE_(n) is concluded by a paced beat, whereT_(n) is the newly computed value of the indicated timing interval,T_(n−1) is the previous value of the indicated timing interval, EE_(n)is the measured timing interval between any two events, and A, B, C, andD are coefficients.

CONCLUSION

The above-described system provides, among other things, a cardiacrhythm management system including techniques for computing an indicatedpacing interval, AV delay, or other timing interval. In one embodiment,a variable indicated pacing interval is computed based at least in parton an underlying intrinsic heart rate. The indicated pacing interval isused to time the delivery of biventricular coordination therapy evenwhen ventricular heart rates are irregular, such as in the presence ofatrial fibrillation. In another embodiment, a variable filter indicatedAV interval is computed based at least in part on an underlyingintrinsic AV interval. The indicated AV interval is used to time thedelivery of atrial tracking biventricular coordination therapy whenatrial heart rhythms are not arrhythmic. Other indicated timingintervals may be similarly determined. The indicated pacing interval, AVdelay, or other timing interval can also be used in combination with asensor indicated rate indicator.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A method, comprising: obtaining actual timingintervals between cardiac events; computing a first indicated timinginterval based at least on a most recent actual timing interval durationand a previous value of the first indicated timing interval, includingweighting at least one of (1) the most recent actual timing intervalduration, or (2) the previous value of the first indicated timinginterval, based on whether the most-recent actual timing interval isconcluded by a paced beat or a sensed beat; and providing pacingtherapy, based on the first indicated timing interval.
 2. The method ofclaim 1, in which computing the first indicated timing interval includessumming a first addend based on the most recent actual timing intervalduration and a second addend based on the previous value of the firstindicated timing interval, wherein at least one of the first and secondaddends is different if the most recent actual timing interval isconcluded by an intrinsic beat than if the most recent actual timinginterval is concluded by a paced beat.
 3. The method of claim 1, inwhich computing the first indicated timing interval (T_(n)) is carriedout according to T_(n)=A·EE_(n)+B·T_(n−1) where A and B arecoefficients, EE_(n) is the most recent actual timing interval betweenevents, and T_(n−1) is the previous value of the first indicated timinginterval.
 4. The method of claim 3, in which computing the firstindicated timing interval (T_(n)) is carried out according to:T_(n)=A·EE_(n)+B·T_(n−1) if EE_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·EE_(n)+D·T_(n−1), ifEE_(n) is concluded by a paced beat, where C and D are coefficients. 5.The method of claim 4, in which at least one of A, B, C, and D is afunction of heart rate.
 6. The method of claim 1, in which computing thefirst indicated timing interval (T_(n)) is carried out according toT_(n)=a·w·EE_(n)+(1−w)·T_(n−1) if EE_(n) is concluded by an intrinsicbeat, otherwise is carried out according toT_(n)=b·w·EE_(n)+(1−w)·T_(n−1), if EE_(n) is concluded by a paced beat,where a, b, and w are coefficients, EE_(n) is the most recent actualtiming interval duration, and T_(n−1) is the previous value of the firstindicated timing interval.
 7. The method of claim 6, in which a isapproximately between 1.0 and 2.0, b is approximately between 1.0 and3.0, and w is approximately between 0 and
 1. 8. The method of claim 1,in which providing pacing therapy is also based on a second indicatedtiming interval that is based on a sensor.
 9. The method of claim 1, inwhich the first indicated timing interval is limited by at least one ofa maximum value and a minimum interval value.
 10. The method of claim 1,in which providing pacing therapy is based on the first indicated timinginterval if atrial tachyarrhythmia is present, and providing pacingtherapy is independent of the first indicated timing interval if noatrial tachyarrhythmia is present.
 11. The method of claim 1, furtherincluding providing to a first ventricle a pace pulse triggered by asensed beat in a second ventricle different from the first ventricle.12. A method comprising: obtaining actual timing intervals betweencardiac events; computing a first indicated timing interval based atleast on a most recent actual timing interval duration and a previousvalue of the first indicated timing interval; and providing pacingtherapy, based on the first indicated timing interval, in whichcomputing the first indicated timing interval is also based on aremotely user-programmable parameter that corresponds to a desireddegree of paced beats vs. sensed beats.
 13. The method of claim 12, inwhich computing the first indicated timing interval includes summing afirst addend based on the most recent actual timing interval durationand a second addend based on the previous value of the first indicatedtiming interval, wherein at least one of the first and second addends isdifferent if the most recent actual timing interval is concluded by anintrinsic beat than if the most recent actual timing interval isconcluded by a paced beat.
 14. The method of claim 12, in whichcomputing the first indicated timing interval (T_(n)) is carried outaccording to T_(n)=A·EE_(n)+B·T_(n−1) where A and B are coefficients,EE_(n) is the most recent actual timing interval between events, andT_(n−1) is the previous value of the first indicated timing interval.15. The method of claim 14, in which computing the first indicatedtiming interval (T_(n))is carried out according to:T_(n)=A·EE_(n)+B·T_(n−1) if EE_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·EE_(n)+D·T_(n−1), ifEE_(n) is concluded by a paced beat, where C and D are coefficients. 16.The method of claim 15, in which at least one of A, B, C, and D is afunction of heart rate.
 17. The method of claim 12, in which computingthe first indicated timing interval (T_(n)) is carried out according toT_(n)=a·w·EE_(n)+(1−w)·T_(n−1) if EE_(n) is concluded by an intrinsicbeat, otherwise is carried out according toT_(n)=b·w·EE_(n)+(1−w)·T_(n−1), if EE_(n) is concluded by a paced beat,where a, b, and w are coefficients, EE_(n) is the most recent actualtiming interval duration, and T_(n−1) is the previous value of the firstindicated timing interval.
 18. The method of claim 17, in which a isapproximately between 1.0 and 2.0, b is approximately between 1.0 and3.0, and w is approximately between 0 and
 1. 19. The method of claim 12,in which providing pacing therapy is also based on a second indicatedtiming interval that is based on a sensor.
 20. The method of claim 12,in which the first indicated timing interval is limited by at least oneof a maximum value and a minimum interval value.
 21. The method of claim12, in which providing pacing therapy is based on the first indicatedtiming interval if atrial tachyarrhythmia is present, and providingpacing therapy is independent of the first indicated timing interval ifno atrial tachyarrhythmia is present.
 22. A method comprising: obtainingactual timing intervals between cardiac events; computing a firstindicated timing interval based at least on a most recent actual timinginterval duration and a previous value of the first indicated timinginterval; and providing pacing therapy, based on the first indicatedtiming interval, in which computing the first indicated timing intervalis also based on a goal proportion of paced beats to sensed beats. 23.The method of claim 22, in which computing the first indicated timinginterval includes summing a first addend based on the most recent actualtiming interval duration and a second addend based on the previous valueof the first indicated timing interval, wherein at least one of thefirst and second addends is different if the most recent actual timinginterval is concluded by an intrinsic beat than if the most recentactual timing interval is concluded by a paced beat.
 24. The method ofclaim 22, in which computing the first indicated timing interval (T_(n))is carried out according to T_(n)=A·EE_(n)+B·T_(n−1) where A and B arecoefficients, EE_(n) is the most recent actual timing interval betweenevents, and T_(n−1) is the previous value of the first indicated timinginterval.
 25. The method of claim 24, in which computing the firstindicated timing interval (T_(n)) is carried out according to:T_(n)=A·EE_(n)+B·T_(n−1) if EE_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·EE_(n)+D·T_(n−1), ifEE_(n) is concluded by a paced beat, where C and D are coefficients. 26.The method of claim 24, in which at least one of A, B, C, and D is afunction of heart rate.
 27. The method of claim 22, in which computingthe first indicated timing interval (T_(n)) is carried out according toT_(n)=a·w·EE_(n)+(1−w)·T_(n−1) if EE_(n) is concluded by an intrinsicbeat, otherwise is carried out according toT_(n)=b·w·EE_(n)+(1−w)·T_(n−1), if EE_(n) is concluded by a paced beat,where a, b, and w are coefficients, EE_(n) is the most recent actualtiming interval duration, and T_(n−1) is the previous value of the firstindicated timing interval.
 28. The method of claim 27, in which a isapproximately between 1.0 and 2.0, b is approximately between 1.0 and3.0, and w is approximately between 0 and
 1. 29. The method of claim 22,in which providing pacing therapy is also based on a second indicatedtiming interval that is based on a sensor.
 30. The method of claim 22,in which the first indicated timing interval is limited by at least oneof a maximum value and a minimum interval value.
 31. The method of claim22, in which providing pacing therapy is based on the first indicatedtiming interval if atrial tachyarrhythmia is present, and providingpacing therapy is independent of the first indicated timing intreval ifno atrial tachyarrhythmia is present.
 32. A method, comprising:obtaining V-V intervals between ventricular beats; computing a firstindicated pacing interval based at least on a most recent V-V intervalduration and a previous value of the first indicated pacing interval;and providing pacing therapy to first and second ventricles, based onthe first indicated pacing interval.
 33. The method of claim 32, inwhich obtaining V-V intervals between ventricular beats includesobtaining the V-V intervals between ventricular beats of the sameventricle.
 34. The method of claim 32, in which obtaining V-V intervalsbetween ventricular beats includes obtaining the V-V intervals betweenventricular beats of different ventricles.
 35. The method of claim 32,in which obtaining V-V intervals between ventricular beats includes:obtaining an initiating ventricular beat, in one of the first and secondventricles, the initiating ventricular beat initiating a V-V interval;and obtaining a concluding ventricular beat, in one of the first andsecond ventricles, the concluding ventricular beat being the nextventricular beat detected after expiration of a first time delay, andthe concluding ventricular beat concluding the V-V interval.
 36. Themethod of claim 32, in which computing the first indicated pacinginterval includes differently weighting at least one of (1) the mostrecent V-V interval duration, or (2) the previous value of the firstindicated pacing interval, if the most-recent V-V interval is concludedby a paced beat than if the most recent V-V interval is concluded by asensed beat.
 37. The method of claim 32, in which computing the firstindicated pacing interval includes summing a first addend based on themost recent V-V interval duration and a second addend based on theprevious value of the first indicated pacing interval, wherein at leastone of the first and second addends is different if the most recent V-Vinterval is concluded by an intrinsic beat than if the most recent V-Vinterval is concluded by a paced beat.
 38. The method of claim 32, inwhich computing the first indicated pacing interval (T_(n)) is carriedout according to T_(n)=A·VV_(n)+B·T_(n−1), where A and B arecoefficients, VV_(n) is the most recent V-V interval duration, andT_(n−1) is the previous value of the first indicated pacing interval.39. The method of claim 38, in which computing the first indicatedpacing interval (T_(n)) is carried out according to:T_(n)=A·VV_(n)+B·T_(n−1), if VV_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·VV_(n)+D·T_(n−1), ifVV_(n) is concluded by a paced beat, where C and D are coefficients. 40.The method of claim 39, in which at least one of A, B, C, and D is afunction of heart rate.
 41. The method of claim 32, in which computingthe first indicated pacing interval (T_(n)) is carried out according toT_(n)=a·w·VV_(n)+(1−w)·T_(n−1)if VV_(n) is concluded by an intrinsicbeat, otherwise is carried out according toT_(n)=b·w·VV_(n)+(1−w)·T_(n−1), if VV_(n) is concluded by a paced beat,where a, b, and w are coefficients, VV_(n) is the most recent V-Vinterval duration, and T_(n−1) is the previous value of the firstindicated pacing interval.
 42. The method of claim 32, in whichproviding pacing therapy is also based on a second indicated pacinginterval that is based on a sensor.
 43. The method of claim 32, in whichproviding pacing therapy is based on the first indicated pacinginterval, if an atrial tachyarrhythmia is present, and providing pacingtherapy is independent of the first indicated pacing interval if noatrial tachyarrhythmia is present.
 44. The method of claim 43, in whichthe atrial tachyarrhythmia is atrial fibrillation.
 45. The method ofclaim 32, in which providing pacing therapy is based on the firstindicated pacing interval if the first indicated pacing interval islonger than a first predetermined value, and providing pacing therapy isindependent of the first indicated pacing interval if the firstindicated pacing interval is shorter than the first predetermined value.46. The method of claim 32, in which computing the first indicatedpacing interval is also based on a remotely user-programmable parameterthat corresponds to a desired degree of paced ventricular beats vs.sensed ventricular beats.
 47. The method of claim 32, in which computingthe first indicated pacing interval is also based on a goal proportionof paced ventricular beats to sensed ventricular beats.
 48. The methodof claim 32, in which the first indicated pacing interval is limited byat least one of a maximum interval value and a minimum interval value.49. A cardiac rhythm management system, comprising: a sensing circuitfor sensing events; a controller, obtaining timing intervals betweenevents and computing a first indicated timing interval based at least ona most recent actual timing interval duration and a previous value ofthe first indicated timing interval; a filter that computes the firstindicated timing interval by weighting at least one of (1) the mostrecent timing interval duration, or (2) the previous value of the firstindicated timing interval, based on whether the most recent actualtiming interval is concluded by a paced beat or a sensed beat; and atherapy circuit, providing pacing therapy based on the first indicatedtiming interval.
 50. The system of claim 49, in which the filterincludes coefficients A, B, C, and D, and the filter computes the firstindicated timing interval (T_(n)) according to: T_(n)=A·EE_(n)+B·T_(n−1)if EE_(n) is concluded by an intrinsic beat, otherwise is carried outaccording to T_(n)=C·EE_(n)+D·T_(n−1), if EE_(n) is concluded by a pacedbeat, where EE_(n) is the most recent actual timing interval durationand T_(n−1) is the previous value of the first indicated timinginterval.
 51. The system of claim 49, further including a sensor, and inwhich the controller computes a second indicated timing interval basedon signals received from the sensor, and in which the therapy circuitprovides pacing therapy that is also based on the second indicatedtiming interval.
 52. The system of claim 51, further including aselection module, selecting between the first and second indicatedtiming intervals to provide a selected timing interval.
 53. The systemof claim 49, including a programmer remote from and communicativelycoupled to the controller, and in which the controller includes a firstparameter that is programmable by the programmer, and the programmerincludes an indicator based on a second parameter received from thecontroller.
 54. A cardiac rhythm management system, comprising: asensing circuit for sensing events; a controller, obtaining timingintervals between events and computing a first indicated timing intervalbased at least on a most recent actual timing interval duration and aprevious value of the first indicated timing interval, and in which thefirst indicated timing interval is also based on a first parameter thatcorresponds to a desired proportion of paced ventricular beats to sensedventricular beats; and a therapy circuit, providing pacing therapy basedon the first indicated timing interval.
 55. The system of claim 54, inwhich the filter includes coefficients A, B, C, and D, and the filtercomputes the first indicated timing interval (T_(n)) according to:T_(n)=A·EE_(n)+B·T_(n−1) if EE_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·EE_(n)+D·T_(n−1), ifEE_(n) is concluded by a paced beat, where EE_(n) is the most recentactual timing interval duration and T_(n−1) is the previous value of thefirst indicated timing interval.
 56. The system of claim 54, furtherincluding a sensor, and in which the controller computes a secondindicated timing interval based on signals received from the sensor, andin which the therapy circuit provides pacing therapy that is also basedon the second indicated timing interval.
 57. The system of claim 56,further including a selection module, selecting between the first andsecond indicated timing intervals to provide a selected timing interval.58. A cardiac rhythm management system, comprising: a sensing circuitfor sensing events; a controller, obtaining timing intervals betweenevents and computing a first indicated timing interval based at least ona most recent actual timing interval duration and a previous value ofthe first indicated timing interval, and in which the controllerincludes a goal proportion of paced ventricular beats to sensedventricular beats, and the first indicated timing interval is also basedon the goal; and a therapy circuit, providing pacing therapy based onthe first indicated timing interval.
 59. The system of claim 58, inwhich the filter includes coefficients A, B, C, and D, and the filtercomputes the first indicated timing interval (T_(n)) according to:T_(n)=A·EE_(n)+B·T_(n−1) if EE_(n) is concluded by an intrinsic beat,otherwise is carried out according to T_(n)=C·EE_(n)+D·T_(n−1), ifEE_(n) is concluded by a paced beat, where EE_(n) is the most recentactual timing interval duration and T_(n−1) is the previous value of thefirst indicated timing interval.
 60. The system of claim 58, furtherincluding a sensor, and in which the controller computes a secondindicated timing interval based on signals received from the sensor, andin which the therapy circuit provides pacing therapy that is also basedon the second indicated timing interval.
 61. The system of claim 60,further including a selection module, selecting between the first andsecond indicated timing intervals to provide a selected timing interval.62. A cardiac rhythm management system, comprising: at least oneventricular sensing circuit; a controller, the controller including: anV-V interval timer, obtaining V-V intervals between successive events inat least one ventricle; a first register, for storing a first indicatedpacing interval; and a filter, updating the first indicated pacinginterval based on a most recent V-V interval provided by the VV intervaltimer and previous value of the first indicated pacing interval storedthe first register; and a ventricular therapy circuit, providing pacingtherapy to first and second ventricles based at least partially on thefirst indicated pacing interval.
 63. The system of claim 62, in whichthe filter differently weighting at least one of (1) the most recent VVinterval duration, or (2) the previous value of the first indicatedpacing interval, if the most recent AV interval is concluded by a pacedbeat than if the most recent AV interval is concluded by a sensed beat.64. The system of claim 63, in which the filter includes coefficients A,B, C, and D, and the filter computes the first indicated pacing interval(T_(n)) according to: T_(n)=A·VV_(n)+B·T_(n−1) if VV_(n) is concluded byan intrinsic beat, otherwise is carried out according toT_(n)=C·VV_(n)+D·T_(n−1), if VV_(n) is concluded by a paced beat, whereVV_(n) is the most recent VV interval duration and T_(n−1) is theprevious value of the first indicated pacing interval.
 65. The system ofclaim 62, further including a sensor, and in which the controllercomputes a second indicated pacing interval based on signals receivedfrom the sensor, and in which the therapy circuit provides pacingtherapy that is also based on the second indicated pacing interval. 66.The system of claim 62, further including an atrial sensing circuit, andin which the filter includes a switch that enables the filter when theatrial sensing circuit indicates that an atrial tachyarrhythmia ispresent.
 67. The system of claim 62, including a programmer remote fromand communicatively coupled to the controller, and in which thecontroller includes a first parameter that is programmable by theprogrammer, and the programmer includes an indicator based on a secondparameter received from the controller.
 68. The system of claim 67, inwhich the first indicated pacing interval is also based on the remotelyuser-programmable first parameter, wherein the first parametercorresponds to a desired degree of paced ventricular beats vs. sensedventricular beats.
 69. The system of claim 62, in which the controllerincludes a goal proportion of paced ventricular beats to sensedventricular beats, and the first indicated pacing interval is also basedon the goal.
 70. The system of claim 62, further including a firstelectrode associated with the first ventricle and a second electrodeassociated with the second ventricle.
 71. The system of claim 62, inwhich the first indicated pacing interval is limited by at least one ofa maximum interval value and a minimum interval value.